Archive for the ‘Mercer Model 35J Raceabout’ tag

Of all the things machinists do to an engine, perhaps the thing that laypeople understand the least is the process of honing. So David Greenlees thought he’d show us how it’s done, using the 1914 Mercer Model 35J Raceabout’s engine for illustrative purposes, then took on the process of machining entirely new valve guides for the Mercer.

When we last finished off we rebored the cylinders and talked about honing them. The photos show the hone in use run wet with honing oil. The next photo shows the final finish, which has the correct cross hatch pattern to help seat the new rings and for oil retention.

The valve guides start out as centrifugally cast gray iron stock, which is machined in the lathe to size. They then get drilled and reamed .005 undersize and finish honed on a Sunnen honing machine wet with oil to a very fine finish.

In this application – since the valve springs, guide and bottom of the valve stem run out in the open – the intake clearance is .002 and the exhaust .004. Cast iron works well as a valve guide especially in this type of use as they are not oiled at all by the engine and only if the driver remembers. Cast iron is very hard and contains graphite which helps somewhat. I also have the valve stems hard chrome plated for durability. A bronze valve guide used this way would gall and stick to the valve.

A do-it-yourself attitude only gets you so far. For example, you can rebuild your entire car, but unless you have the specialized equipment to bore and hone your cylinders, you’re going to have to rely on somebody else to do it for you. But it doesn’t mean you have to remain ignorant about the process, so our friend David Greenlees thought he’d shed a little light on it, using the cylinders from a 1914 Mercer Model 35J Raceabout as an example.

This may give you a better idea of what to look for when you have to get the cylinders rebored for a project.

Many things can go wrong with cylinders especially if they were machined incorrectly at the factory which happens sometimes with mass production. Or they can also have been rebored incorrectly.

This engine has removable cylinders which was common up until about 1930 but they do not have a removable head. If the top of the crankcase where these cylinders attach to is parallel to the crankshaft (it’s checked with precision measuring tools) and they are flat and not warped on the bottom and set up to be rebored on a precise fixture and checked, the bore will be perfectly perpendicular to the crank. This is what you need to have done and be willing to find the right shop who can do this.

If the bores are not perpendicular the pistons will scuff, the bore will score and the rings will never seal right. The engine will be down on power, burn oil, run hot, have a lot of blow by and wear the rod bearings.

If you have a one-piece block with removable heads look for a shop that can set up your block off of the main bearing bores and then bore it (the top surface of the block is often off and if bored from there you may have problems).

This fixture has been precision ground and is perfectly parallel. The block is clamped to the bottom and measured with a micrometer to be sure of parallelism and the boring bar attaches to the top and is also checked. This bar is a 1930′s Van Norman which can bore up to 7.5″ x 18″ deep. It works better than most modern bars and leaves and incredibly smooth surface. They are unbelievably well built and have pads that expand out and steady the bar to the bore which make for the ultra smooth finish. Check out the neat old plate on it. After boring the final five thousandths is honed out with a rigid Sunnen hone which leaves the right finish for ring seating and oil retention.

Do not let a shop talk you into honing your block out, make sure they can bore it correctly and then finish hone it.

Cams and bearings are often over looked and this is what is involved when you what it done correctly and parts are not available.

The first thing is to check and straighten the cam as they often bend after use. The cam bearing journals are then ground and polished. The bearings were originally die cast babbitt but the new ones we made are of bushing bronze which has a high percentage of lead and tin it that make for a good bearing.

The center bearings are split so first the new one gets roughed out in the lathe and then split in half in the milling machine with a slitting saw. The parting faces are then milled flat and the two half’s are then soldered together with two hose clamps tightly squeezing them together.

The O.D. and the I.D are then finished machined in the lathe and unsoldered to make a perfect bearing. The bearings are keyed to the bore in the case so a slot is machined and a bronze key silver soldered in. The bearings are also a .0005 light interference fit into the case. Oil slots are also machined in for the supply. The front bearing is a ball bearing and the rear bearings are flanged and full round so they are much easier to machine.

One other problem is that the intake cam has a removable end which serves as the bearing journal and also drives the oil pump. It was a tight wringing fit driven by a key but it use had worn to .006 thousandths clearance. The I.D. of the end which is hardened was ground out .015 larger. The end of the cam was machined down .015. A sleeve was machined out of pre-hardened 4140 and pressed on. The O.D. was then ground to a fit of only .0003 and the key way opened back up. The removable end had a damaged center which was necessary to hold it in the lathe so that the O.D. of the journal could be ground with both parts assembled.

A stub was machined true in the lathe and the removable end lightly heated up to expand it slightly and slipped on. Since the end was hardened and the center not machinable a Packard cylinder head acorn nut was tightly put on to the threaded end.

It was them machined flat and a new center put on to it. The removable end was then pressed off of the stub and assembled onto the cam and the bearing journal was ground in the lathe so that they would both run true when assembled.

While speaking with David Greenlees at the New England Concours over the weekend, he said he was able to roll the 1914 Mercer Model 35J Raceabout out of the garage over the weekend while preparing the 1915 Duesenberg racer for the Concours. As you can see, it’s not quite roadworthy yet, but it’s coming along nicely now that it’s off the jackstands.

Note that he hasn’t yet reinstalled the gas tank. That’s because it’s still trussed up for cleaning, as David showed us earlier.

He described it as the “Rube Goldberg gas tank cleaning machine.” Belted to the lathe and with ceramic media inside, it seems to work fairly well.